Castings



United States Patent 3,496,624 CASTlNGS David L. Kerr, Cleveland, RobertC. Lemon, Lakewood, and Edward E. Stonebrook, Cleveland, Ohio, assignorsto Aluminum Company of America, Pittsburgh, Pa., a corporation ofPennsylvania No Drawing. Filed Oct. 25, 1966, Ser. No. 589,236 Int. C].1523 17/00 US. Cl. 29-1962 6 Claims ABSTRACT OF THE DISCLOSURE Fatiguestrength of aluminum or aluminum alloy castings is improved bysubjecting them to isostatic pressure at an elevated temperature for asufficient time to heal micropores.

This invention relates to aluminum or aluminum alloy castings havingimproved fatigue strength and to a process for providing such castings.

In fabricating different members or shapes of aluminum or aluminumalloys, particularly intricate shapes, casting is almost invariablyconsidered, in those cases where there is any choice, because of costsavings over forging and other methods. However, aluminum or aluminumalloy castings are marked by certain disadvantages which limit theiruse. One such limitation inherent in such castings as commerciallyproduced is that their fatigue strength is significantly inferior tothat of forgings. By way of illustration, a forging is generallyconsidered to exhibit a fatigue strength of 25%, or more, greater thanthat of a corresponding casting. By fatigue strength, in thisdescription, is meant the stress which can be applied for apredetermined number of cycles Without failure. In the aluminum industryit is common to designate as the fatigue strength the stress withstoodfor five hundred million (x10 cycles without failure.

Accordingly, it is an object of the invention to provide aluminum oraluminum alloy castings having improved fatigue strength, ,on the orderof 25 percent or more.

According to the invention, aluminum or aluminum alloy castings areimproved in fatigue strength by subjecting them to a sustained,substantially isostatic, pressure application of at least 3000 p.s.i. ata temperature of at least 600, preferably at a temperature of 700 to1000 F. By substantially isostatic pressure is meant substantially equalpressure from every side, analogous to the hydrostatic pressure on abody submerged in a liquid at rest. While the pressure should be atleast 3000 p.s.i., a range of 5000 p.s.i. and higher, for example 10,000to 100,000 p.s.i., is usually preferred. Temperatures slightly higherthan those stated are permissible although the temperature should not beso high as to cause melting. Lower temperatures are preferably avoidedsince a consistent improvement cannot be achieved without substantialdifficulty. It can be seen from the foregoing that the inventioninvolves subjecting the casting to temperature and pressure levels whichare normally applied to effect substantial metal movement. However, inpracticing the invention such metal movement is substantially avoided.

An important feature in practicing the invention is that the isostaticpressure is imposed on the entire casting for a sustained length oftime, for example several seconds to 1 hour or longer. This can becontrasted to the situation occurring in a forging operation where agiven pressure level usually is not sustained by the entire member andusually is not maintained for any significant duration. In forging, thepressure application is either very brief, as in impact forging, orimmediately relieved by metal movement or both. In the practice of theinvention, the minimum time period for pressure application can beconsidered as somewhat dependent on the isostatic pressure level, ashigher isostatic pressures may be maintained advantageously for shorterlengths of time than lower pressures. Temperature is another relatedfactor, as higher temperatures tend to facilitate the advantageous useof lower pressures, shorter periods of pressure application, or both.

Various means may be employed for applying the isostatic pressure inpracticing the invention. The most convenient means are mechanicalpressure application, as by dies, and fluid pressure application, as bya gas. In mechanically applying the isostatic pressure, the casting isplaced within dies shaped to conform to the casting. Pressure can thenbe applied by any means such as a hydraulic press. It is important thatthe dies be so constructed as to substantially prevent any metal flow inthe casting, i.e. the casting must be confined so as to substantiallyprevent any significant movement thereof. Thus conventional forging dieswould generally have to be modified somewhat with respect to flashplanes, or other provision for metal extruding out of the die cavity, assuch would relieve the isostatic pressure by metal movement. Since theisostatic pressure which can be applied mechanically is limited only bythe press capacity, rather large isostatic pressures can be appliedmechanically. For instance, pressures of 20,000 to 50,000 p.s.i., oreven much greater pressures of up to 100,000 p.s.i. or more, can beprovided using conventional presses. Because of the isostatic pressureseasily achieved in mechanical pressure application, the duration of theapplication often can be relatively short, for example 15 seconds, or insome cases even less, although to assure consistently good results it isadvisable to maintain the pressure application for 15 seconds or more.

In fluid isostatic pressure application, the casting may be placed in achamber which is pressurized to a level corresponding to the isostaticpressure desired while the casting is maintained at a temperature asherein provided. Because of present economic limitations, theautoclaves, or the like, suitable for such simultaneous temperature andpressure application and large enough to handle commercial castings, areoften limited to pressure levels of 20,000 p.s.i. or less. Because thesepressures are relatively low within the context of the improved process,the duration .of pressure application is lengthened somewhat over thatpermissible with higher pressures.

Castings subjected to sustained temperature and isostatic pressureapplication in accordance with the foregoing exhibit improved fatiguestrength over like castings not so treated. The improvement observed inboth sand and permanent mold castings is generally at least 25 percent,although improvements of 50 to percent, or more, are often realized.This is considered the most significant change in properties over thoseof the untreated casting. While tensile properties may often be improvedto some extent, such is considered incidental in comparison to thepronounced improvement in fatigue strength. 7

The improved castings retain the basic qualities of a cast internalstructure. Micrographs taken before and after pressure applications showthat there is no discernible deformation of the grains, and that thecastings retain their characteristic random grain size, shape andorientation after isostatic pressure application. The isotropicproperties characteristic of a cast internal structure, i.e. generallyuniform stress and general corrosion resistance, strength, and otherproperties, in all directions, are likewise retained.

One difference which is observed in metallographic examination of theimproved castings, however, is that microporosity is substantiallyeliminated in its entirety. It

is generally recognized that aluminum or aluminum alloy casting productswill exhibit some degree of microporosity regardless of the care that istaken to avoid such. The improved castings exhibit substantiallycomplete freedom from micropores over 0.0001 inch, in size, a sizereadily discernible at 500x magnification in castings not treated inaccordance with the invention. By 0.0001 inch in size is meant that thelargest dimension does not exceed this value. We have found thatsubstantially eliminating the micro-pores, in the neighborhood of 0.0001to 0.001 inch, significantly increases the fatigue strength of aluminumcastings. Castings improved in accordance with the invention, havingtheir internal structure derived from subjecting such to the isostaticpressure application described herein, are substantially free frommicropores over 0.0001 inch in size, the internal structure otherwisebeing substantially as cast, and they exhibit markedly improved fatiguestrength, by as much as 25% or more. It is believed that the improvementin fatigue strength is related to elimination of microporosity asdescribed, and accordingly the duration for which the temperature andisostatic pressure is sustained should be sufficient to render thecasting substantially free from such microporosity.

There is no particular aluminum or aluminum alloy composition limitationto which the invention is confined. It is particularly useful formoderate to high strength aluminum alloys generally employed in the moredemanding applications, for instance heat treatable aluminum castingalloys. The temperature and isostatic pressure application does notaffect the response of the casting to solution heat treatment. Solutionand precipitation or other heat treatments may be employed without lossof the benefit conferred by the practice of the invention. In fact,because the temperature range employed in the isostatic pressureapplication may approximate that used in solution heat treating, thecasting can be quenched immediately after isostatic pressure applicationso as to retain in solution the alloying constituents dissolved duringthe thermal exposure. An artificial aging treatment can then follow.Obviously a separate heat treatment can be applied after isostaticpressure application. The casting may be machined at any stage before orafter isostatic pressure application. For instance, a rough sand castingmay be machined, improved as herein provided, heat treated and thenfinish machined, if desired.

The following are illustrative examples of the invention.

EXAMPLE 1 A full skirted cast diesel engine piston casting of analuminum alloy containing, nominally, 4 /2 copper, 1 /2% magnesium and2% nickel was produced in a permanent mold. The piston casting measuredabout 5 /2 inches in diameter by 6 /2 inches in height. A die wasconstructed to receive the piston casting so that, except for the topsurface of the piston head, the piston mated closely with the diecavity. The casting was heated to 850 F. and placed in the die which hadbeen preheated. A flatfaced ram was inserted into the open end of thedie so that it could bear against the fiat top surface of the pistonhead. The ram, covering the entire piston head, was very closely fittedwith the die cavity so as to minimize any metal leakage between the ramand the die cavity. The ram was pressed into the die by a 500 tonhydraulic press for one-half minute, imposing a pressure of over 47,000p.s.i. on the piston. This pressure was substantially isostatic, asthere was practically no metal movement. Since the die cavity and pistonwere fitted close, the amount of flash observed at the piston head waspractically nil. Micrographs further verified the lack of significantmetal movement as there was no discernible grain distortion. Themicrographs also indicated complete freedom from any micropores over0.0001 inch in size. Several such pistons were cut into sections fromwhich R. R. Moore type rotating-beam fatigue specimens were machined.The

TABLE 1 Fatigue strength,

p.s.i., Tensile Yield 5x10 strength, strength, Percent; Condition cyclesp.s.i. p.s.i. El. 111 2 As cast 9, 000 27, 000 26, 000 0. 7 Improved 17,000 33, 000 26, 000 1. 6

It can be seen that the fatigue strength of the improved castings wasalmost double that of the unimproved castings. Some incidental increasein tensile properties is also evident.

EXAMPLE 2 A partially skirted piston casting having a piston ring insertcast in place in the vicinity of its upper periphery was improved inaccordance with the process set forth herein. The band-like ring insertwas composed of the well-known Ni-Resist cast iron-nickel alloy oftenemployed for this purpose. With the exception of the ring insert, thegeneral size and alloy composition of the piston were identical to thatof the fully skirted piston described in Example 1. A die was providedwith a mating ram member as in Example 1. Each casting was heated to 850F., placed in the heated die and the ram was pressed into the dieagainst the piston head for /2 minute by a 700 ton hydraulic press whichimposed a substantially isostatic pressure of approximately 54,000p.s.i. Fatigue specimens were removed from the wrist pin boss of severalpistons treated as just described. The fatigue tests indicated that thecast aluminum alloy portion of the composite piston structure wasimproved to the same extent as that of pistons made entirely of aluminumalloy as described in Example 1. This is a particularly importantembodiment of the invention, since ferrous ring inserts are oftenincluded in commercial aluminum pistons, especially where heavilyloaded. Prior to this invention the inclusion of such an insert,metallurgically bonded in place, was effected by casting the aluminumpiston around the insert, but such pistons were characterized byrelatively low fatigue strength, especially in the critical wrist pinboss area. Several schemes evolved for forging the piston around theinsert but such results in a bond considerably inferior to themetallurgical bond achieved where the insert is cast in place. A furtherproblem occurs in that the forging operation often imposes excessstresses on the ring member causing it to distort, crack, or worse, faillater in service. The improved pistons retain the advantages of the castin place insert structure while exhibiting markedly improved fatiguestrength previously associated only with forged pistons.

EXAMPLE 3 A cylindrical cast iron permanent mold was used to produce 6inch diameter by 8 inch long castings of an aluminum alloy containing,nominally, 9% silicon, /2% magnesium and 1.8% copper. From these, 5 /2inch diameter by 2 /2 inch long disc-like specimens were cut andmachined. The specimens were heated to 800850 F. and placed in a closelyfitted preheated die. A load of 500 tons was applied for one-half minuteby a hydraulic press to the top of each specimen. The specimens werereheated to 980 F. and held at that temperature for 8 hours and thenquenched in water at F. followed by artificial aging for 10 hours at 310F. Fatigue specimens of the type described in Example 1 were preparedfrom these.

and from specimens which had not been subjected to the pressureapplication but which were otherwise identical. The fatigue tests inthis instance were conducted at a stress of 20,000 p.s.i. and thecomparative specimen life extrapolated to the fatigue strength at 5 10cycles. Again tensile properties were also compared. The results ofthese tests are listed in Table 2, where it is evident that the improvedspecimens exhibit an improvement in fatigue strength of 33% over theas-cast material.

Cast pistons of the same size and composition as set forth in Example 1were placed in an autoclave and subjected to a helium gas pressure of15,000 p.s.i. at a temperature of about 900 F. for about 2 hours. Theresults of R. R. Moore type fatigue tests on specimens removed from thewrist pin bosses of these pistons indicated a log mean fatigue life of1.5 X cycles at a stress of 20,000 p.s.i. which is extrapolated toindicate a fatigue strength of 15,000 p.s.i. for 5 10 cycles. That is,nearly the same improvement was realized whether using mechanical as inExample 1, or fluid isostatic pressure application.

What is claimed is:

1. The method of improving an aluminum or aluminum alloy castingcomprising subjecting the casting to a substantially isostatic pressureof at least 3,000 p.s.i. While maintaining said casting at a temperatureof at least 600 F. but less than the melting temperature thereof, for asufficient time to render the casting substantially free from microporesover 0.0001 inch in size, said casting being characterized bysubstantial increase in fatigue strength over a like casting notsubjected to said substantially isostatic pressure application.

2. The method of improving the fatigue strength of an aluminum oraluminum alloy casting comprising subjecting the casting to asubstantially isostatic pressure of 10,000 to 100,000 p.s.i. at atemperature of 700 to 1000" 6 F. for a suflicient time to render thecasting substantially free from micropores over 0.0001 inch in size,said casting being characterized by a substantial increase in fatiguestrength over a like casting not subjected to said substantiallyisostatic pressure application.

3. The method according to claim 1 wherein the substantially isostaticpressure is from 5000 to 20,000 p.s.i., applied by the action of afluid, and is maintained for at least one hour.

4. The method according to claim 1 wherein the sub stantially isostaticpressure is from 20,000 to 100,000 p.s.i., applied by dies conforming tothe shape of the casting, and is maintained for at least fifteenseconds.

5. An improved aluminum or aluminum alloy casting having a cast internalstructure substantially free of micropores over 0.0001 inch in size andderived from subjecting an aluminum or aluminum alloy casting to asubstantially isostatic pressure of at least 3000* p.s.i. at atemperature of at least 600 F. but less than the melting temperaturethereof for a sutficient time to render said cast internal structuresubstantially free of said micropores, said aluminum or aluminum alloycasting being characterized by an increase of at least 25 percent infatigue strength over a like casting not subjected to said substantiallyisostatic pressure application.

6. An improved aluminum or aluminum alloy casting according to claim 5which includes a cast in place ferrous portion, metallurgically bondedto the aluminum casting.

References Cited UNITED STATES PATENTS 1,891,234 12/1932 Langenberg148131 X 1,936,652 11/1933 Yeomans 148-131 X 1,946,545 2/1934 Pessel148131 2,672,430 3/1954 Simons 14813l 2,762,734 9/ 1956 Corral 148-131 X2,778,756 1/1957 Bredzs 148131 2,878,140 3/1959 Barr 29182 X 3,157,54011/1964 Bobrowsky 148131 FOREIGN PATENTS 696 1882 Great Britain. 905,61912/ 1945 France.

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R.

